The present invention relates to a transfer optical surface machining method, an optical device producing mold and an optical device. More specifically, this invention relates to a method of machining the transfer optical surface of an optical device producing mold characterized by a high degree of hardness, suitable for a glass mold, to the optical device producing mold manufactured by using the same, and to the optical device produced using the same.
In the prior art, the most popular method of producing an optical device is injection molding of plastic materials. Injection molding has been producing an optical device of higher precision with high efficiency at a lower cost. However, the development of the digital imaging technology has enabled easy handling of high-volume image information, and a rapid advance has taken place in achieving higher definition and miniaturization of an image capturing camera as one of the input devices. There has been an increase in the number of pixels of the image capturing device, and further miniaturization of the device. Thus, the image capturing lens for forming an optical image thereon is required to provide remarkable performances, and the optical device is required to ensure higher precision.
In the optical disk used to input and store the image as well as the information on music and text data, there has been an active demand for higher density, i.e. higher volume and further miniaturization. The wavelength of the laser light source used in the read/write operations on the optical disk is changing from the conventional red to blue and purple of shorter wavelength. To meet this trend, the pickup optical device is required to ensure higher precision and higher NA (numerical aperture) from the conventional NA of 0.65 to a new 0.85 or more. It is further required to provide higher performances and higher functions capable of ensuring compatibility with the conventional optical disk system. Techniques meeting these requirements have been developed, and are being put into practical use.
As described above, there has been a rapid demand for an optical device of the precision and function far advanced over the conventional level. In this context, it is not sufficient that the initial performance of the optical device is high. Because of the improved performances, the optical device is subjected to a drastic change in performances when exposed to changes in the temperature and wavelength. To avoid such a problem, it is very important that the optical device is capable of stable maintenance of initial performances such as environmental resistance and explosion resistance.
The optical glass, produced by hot press molding, called a glass mold (hereinafter referred to as “GM”) is more preferably used such an optical device than plastic material, because the GM is hardly subjected to a change in refraction index, and offers a wider range in the selection of materials, thereby expanding the scope of freedom in designing.
However, the softening temperature of the optical glass is 400 through 600° C. This is a few hundred degrees higher than the softening temperature of the plastic. At this temperature, the mirror surface cannot be maintained in the atmosphere due to oxidation, even in the case of a heat-resistant metallic material. Accordingly, the GM producing mold requires use of such a material as ceramic or cemented carbide characterized by high heat resistance and a high degree of hardness (having a Vickers hardness of Hv 1500 or more) so that the transfer optical surface is hardly damaged. Incidentally, in the production of these optical devices, it goes without saying that the contour precision and surface roughness of the mold serving as a transfer master are major factors in determining the quality of the molded optical device. In the present specification, the term “transfer optical surface” refers to the portion (surface) of the mold for molding and transferring the optical surface of the optical device to be produced.
The high-hardness material of such a GM producing mold is very difficult to process in order to get the transfer optical surface. A diamond is commonly used as a tool for machining such a material. In the conventional method, a wheel using diamond abrasive grains has been employed to perform generative machining of the transfer optical surface by grinding. Polishing has been done after grinding, in some cases.
After machining a high-hardness material, a new cutting blade (diamond abrasive grain) present under the surface of a diamond wheel appears (self-regenerating function of the blade). This mechanism prevents the machining capacity being deteriorated during machining. However, such a wheel is produced by mixing the abrasive grain with the binder, and burning and setting thereafter. Accordingly, a uniform structure cannot be obtained when viewed from a microscopic scale. It is difficult to manufacture a wheel contour with high precision. Accordingly, the machined contour is easily subjected to an error due to the wheel contour error. Further, the binder does not have much hardness, and is subjected to elastic deformation due to the force applied to the wheel during the machining. Thus, on a microscopic scale, grinding does not provide a method of transferring the precision of movement by forced infeed; it rather provides pressure transfer. Machining precision can hardly be improved if the machine movement precision and tool rotary precision have been improved.
Further, when dressing the wheel, a process of cutting the wheel called “truing” is applied. As described above, the wheel is made of a mixture between the very hard diamond abrasive grain and soft binder. Accordingly, uniform cutting is not performed, but very rough machining is performed, wherein the diamond abrasive grains are picked up while the binder is scraped. The wheel contour is dressed in this manner. Thus, the contour precision of the produced wheel is not very satisfactory. Not only that, the binder may be cracked by scraping. Force may have been applied to the diamond abrasive grains to disengage them, and the diamond abrasive grains may be kept unstable. In some cases, the diamond abrasive grains are disengaged so that holes are formed. The surrounding binder is damaged by excessive movement at the time of scraping the binder. Such a phenomenon occurs to the surface of the wheel, which is kept very unstable. Accordingly, in the conventional method, preliminary grinding operation was performed after truing, whereby the unstable portion of the wheel surface is removed, and grinding ratio is stabilized. However, the degree of preliminary grinding for stabilizing the wheel surface has been determined intuitively. The conventional concept of dressing such a tool of rough structure by the aforementioned rough method has been unfitted for high-precision, high-accuracy machining of the transfer optical surface.
Further, grinding is based on the self-regenerating function of the cutting blade resulting from wear of the wheel. The contour of the wheel is constantly subjected to changes during machining. A contour error occurs on the transfer optical surface having been produced. Especially in the concave and small transfer optical surface or the deep transfer optical surface (having a depth), the wheel size must be reduced in order to ensure that the wheel reaches the surface to be machined, without interfering with the outer periphery. Machining of a high-precision transfer optical surface is difficult due to rapid wear of the wheel.
The present applicant found out that cutting by a diamond tool is effective in the generation of a transfer optical surface using a high-hardness material, and has already proposed a method based on this principle. The Patent Document 1 according to the application filed by the present applicant discloses that high-precision machining contour and mirror surface can be provided by cutting; using an optical device producing mold.    [Patent Document 1] Official Gazette of Japanese Patent Tokkai 2004-223700
The technique disclosed in the Patent Document 1 is far superior to the conventional grinding with respect to machined contour precision and surface roughness. Superiority is increased with a decrease in the transfer optical surface diameter. However, chipping of the diamond tool tends to occur, when cutting a material of high hardness using a diamond tool. If the tool is chipped, a uniform and continuous surface cannot be obtained after cutting. The surface roughness will deteriorate and the amount of infeed will be changed. The required contour precision of the transfer optical surface cannot be maintained. Such a problem of chipping may occur to other tools than a diamond tool.